Re: [PATCH v3 1/4] mm: introduce get_user_pages_longterm

[Michal Hocko]

On Fri 01-12-17 08:29:53, Dan Williams wrote:
> On Fri, Dec 1, 2017 at 8:02 AM, Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> >
> > On Fri, Dec 01, 2017 at 11:12:18AM +0100, Michal Hocko wrote:
> > > On Thu 30-11-17 12:01:17, Jason Gunthorpe wrote:
> > > > On Thu, Nov 30, 2017 at 10:32:42AM -0800, Dan Williams wrote:
> > > > > > Who and how many LRU pages can pin that way and how do you prevent nasty
> > > > > > users to DoS systems this way?
> > > > >
> > > > > I assume this is something the RDMA community has had to contend with?
> > > > > I'm not an RDMA person, I'm just here to fix dax.
> > > >
> > > > The RDMA implementation respects the mlock rlimit
> > >
> > > OK, so then I am kind of lost in why do we need a special g-u-p variant.
> > > The documentation doesn't say and quite contrary it assumes that the
> > > caller knows what he is doing. This cannot be the right approach.
> >
> > I thought it was because get_user_pages_longterm is supposed to fail
> > on DAX mappings?
> 
> Correct, the rlimit checks are a separate issue,
> get_user_pages_longterm is only there to avoid open coding vma lookup
> and vma_is_fsdax() checks in multiple code paths.

Then it is a terrible misnomer. One would expect this is a proper way to
get a longterm pin on a page.
 
> > And maybe we should think about moving the rlimit accounting into this
> > new function too someday?
> 
> DAX pages are not accounted in any rlimit because they are statically
> allocated reserved memory regions.

Which is OK, but how do you prevent anybody calling this function on
normal LRU pages?

-- 
Michal Hocko
SUSE Labs